The present invention relates to magnetic head and disk testers, and in particular, to a method and apparatus for improving the accuracy of the mechanisms that position a magnetic head with respect to a magnetic disk in a magnetic head and disk tester.
A magnetic head and disk tester is an instrument that is used for testing characteristics, such as signal-to-noise ratio, track-to-track error, of magnetic heads and disks. In some cases, a disk (e.g., a computer hard drive disk) may be tested with a known and calibrated head and in other cases a magnetic head may be tested with a calibrated disk. A magnetic disk and head tester consists of two main portions, a mechanical portion that performs movements of the head with respect to a disk supported by the tester, and an electronic portion that is responsible for measurement, calculation, and analysis of data. The mechanical portion of the tester is known as a spinstand. The quality of the results of tests performed using such a magnetic head and disk tester depends at least in part upon the positioning accuracy provided by the spinstand for the head with respect to the disk.
A typical magnetic head and disk tester of the prior art is shown schematically in FIGS. 1 and 2, and is described in U.S. Pat. No. 5,382,887 to Guzik, et al. In the description below, spinstands are described in which a carriage (and attached head) is selectively moved along an axis in a horizontal plane, where the head is moved with respect to a horizontally supported disk, rotatable about a vertical axis. While these vertical and horizontal reference directions are used in the exemplary spinstands, other orientations may be used in other embodiments.
The illustrated spinstand 10 includes a base 50 having a disk spindle 46 extending vertically (as illustrated) therefrom. The spindle 46 supports a disk 42 in a horizontal (as illustrated) plane, in a manner permitting controlled rotation of the disk about a vertical axis.
A carriage 30 is slidably supported on a plurality of rails 22, 24 which are rigidly mounted to a base 50, whereby carriage movement can occur along a horizontal axis (X-axis). A head support element 44 is secured to carriage 30 and includes a magnetic read-write head 40 mounted at its distal end. A drive assembly includes (1) a coarse positioner, for effecting gross motion of the carriage 30 along rails 22, 24, and (2) a fine positioner, for effecting minor motions of the carriage 30. The movement of carriage 30 results in a corresponding movement of the magnetic head 40 to desired positions over a magnetic disk 42. The magnetic head 40 is moveable in a radial direction relative to the disk 42, such movement facilitating testing of a disk or a head.
In this prior art systems, the linear position of the carriage 30 relative to base 50 and thus the relative position of the magnetic read-write head 40 to the disk 42, is measured using two linear encoders 12 and 14 that are symmetrically mounted to the carriage 30 on opposite sides of the X-axis. That is, a first encoder 12 is mounted on a right lateral side of the carriage 30 and a second encoder 14 is mounted on a left lateral side of the carriage 30. Outputs of each encoder 12 and 14 are supplied to an arithmetic unit 52 which determines the position of the magnetic read-write head 40 using these outputs. Each encoder is substantially comprised of two parts. One part is secured to the base 50, and so is stationary. The other part is affixed to carriage 30, and so is moveable relative to the stationary part of the encoder. The measurement of the relative movement of the moveable part of the encoder with respect to the stationary part of the encoder is used to determine the movement and position of the head 40 relative to the disk 42.
The coarse positioner of the spinstand includes a stepper motor 32 affixed to base 50, a lead screw 34, a nut 36 on the lead screw 34 and a block 38 which is mounted on rails 26 and 28. The nut 36 is attached to the block 38. The lead screw 34 and nut 36 are used to transfer the rotary movement of the stepper motor 32 to linear movement of the block 38 on rails 26, 28 along X-axis.
The fine positioner of the prior art spinstand 10 includes carriage 30 which is mounted on rails 22 and 24 and moves along the X-axis, and a piezoelectric actuator 48. The piezoelectric actuator 48 is mounted between the block 38 and the carriage 30. The actuator 48 is responsive to voltages applied thereto, to change its dimension in the x-direction, direction, which in turn results in displacement of the carriage 30 with respect to block 38.
In operation, the coarse positioner is able to move the carriage 30 over relatively long distances, but remains limited in linear resolution to the degree that it cannot position the magnetic read-write head 40 with the accuracy required to adequately test current heads and disks. The magnetic recording technology today requires spinstands that can position the magnetic read-write head 40 with an accuracy of about 10 nm or better, thus the need for the fme positioner. The piezoelectric actuator 48 has a much shorter movement range than the coarse positioner, and can position the carriage 30 with the required accuracy of 10 nm. A typical piezoelectric actuator 48 would be PZT-5H produced by Morgan Matroc Inc., Ohio, U.S.A. This unit has a 15 micrometer range and is able to create movement with steps shorter than 10 nm.
The prior art methods used to test the magnetic read-write heads and disks include positioning the magnetic read-write head 40 a number of times with very small displacements that require the accuracy of the fine positioner. These small movements require extreme accuracy in the fine positioner. During these movements, it is common, for the magnetic read-write head 40 to be moved from a predetermined position to a new position, and then return to the first position. A signal read by the magnetic read-write head 40 can reveal any mismatch between the intended position, and the actual position of the magnetic-read-write head 40. The difference in position due to this forward and backward movement of the carriage 30 is called "mechanical hysteresis" and it is possible to detect this hysteresis by measuring the amplitude of the signal read by the magnetic read-write head 40.
There are many causes for mechanical hysteresis, and two of the most common ones in a spinstand are the yaw and pitch of the carriage 30. The term "yaw" refers to angular motion of the carriage 30 about a vertical axis. The term "pitch" refers to angular motion of the carriage 30 about a horizontal axis which is perpendicular to the x-axis.
The position of the carriage 30 is measured using the linear encoders 12, 14 during each coarse and fine positioning movement. In an ideal case, the linear encoders are mounted in close proximity to the magnetic read-write head 40, and therefore would measure its actual position. In reality, due to mechanical, limitations, it is often not possible to position the linear encoders this way. Rather, the encoders are mounted away from the head. This orientation leads to errors in the measurement of the position of the head, due to the pitch and yaw motions of the carriage. Therefore, a plurality of linear encoders need to be used to best determine the actual position of the head. In U.S. Pat. No. 5,382,887, granted Jan. 17, 1995, and assigned to the assignee of the present invention, the yaw motion of a carriage (and attached head) is detected by placing a linear encoder 12 and 14 on each side of the carriage, parallel to the direction of the movement of the carriage and symmetrically with respect to the center line (X-axis) of the carriage. Using the difference in the readout of the two linear encoders 12 and 14, the amount of yaw that occurs during the positioning movements can be determined. However, there is no current method or apparatus for determining the amount of, and accommodating, pitch of the carriage 30.